1. Field of the Invention
The present invention relates to a starter motor for cranking and starting an engine, more particularly to a starter which has a speed reduction mechanism using a planetary gear device.
2. Description of Related Art
Recently, more starters having a planetary gear reduction device which are small in size and light in weight are used for vehicles because of necessity to save a space in an engine compartment and to reduce fuel consumption. In order to reduce the size and weight of the starter, it is desirable to further increase a reduction ratio of the reduction device of the starter. To increase the reduction ratio of the planetary gear reduction device while keeping the size thereof within a predetermined diameter D (shown in FIG. 7) of an internal gear, it is necessary to reduce a Module (M) of gears as shown in FIG. 6.
Generally, a relation between the Module M and a stress .sigma..sub.b at a tooth foot is expressed in a following equation. EQU .sigma..sub.b =P/(M.times.b.times.y)
where y=(cos .alpha..times..phi..sup.2)/(cos .omega..times.6.psi.),
P: Impact force applied to a tooth tip by transmission torque, PA2 M: Module, PA2 b: Gear width, PA2 .alpha.: Pressure angle, PA2 .phi.: Tooth foot width divided by M, PA2 .omega.: Angle made by an operating line of P and a line perpendicular to a tooth center line, and PA2 .psi.: Distance between a tooth foot and a cross-point of an operating line of P and a tooth center line, divided by M.
As seen from the equation, as the Module M is decreased, the stress .sigma..sub.b becomes larger. On the other hand, in a conventional planetary gear reduction device, the impact force P is applied to gear teeth at a gear engagement and during a cranking period as shown in FIG. 8. Therefore, it has been difficult to decrease the Module M without causing damages of teeth due to the large engagement impact and fatigue of the teeth due to a repetition of the cranking impacts. The damages of the gear teeth often occurs to a sun gear of the planetary gear reduction device.
Because of the reason mentioned above, the Module M of conventional planetary gear reduction devices has been chosen to be more than 1.25 and the reduction ratio to be less than 5.45, as exemplified in Japanese Patent Laid-Open Publication No. Hei-2-238171. Because the Module M cannot be reduced, there has been a certain limit in reducing the size and weight of starter motors.
It has been also attempted to increase the reduction ratio of the planetary gear reduction device by using a multi-stage planetary gear system as shown in Japanese Patent Laid-Open Publication No. Hei-6-159205. The multi-stage reduction system, however, cannot make the device small and light.